1. Field of the Invention
The present invention relates generally to methods of manufacturing miniaturized devices such as semiconductor wafers (referred to hereinafter as wafers) and, particularly, it relates to a method for detecting optically an end point of treatment of a layer formed on a surface of a wafer when the layer is treated while being supplied with a treatment solution.
2. Description of the Related Art
An example of the art of this type is disclosed in Japanese Patent Laying-Open No. 62-63431, which is a method of optically detecting an end point of developing a photoresist film. This method utilizes interference of light.
Referring to FIG. 1, the conventional end point detecting method will be described. A photoresist film 2 having a thickness d is applied to a wafer 1. The photoresist film 2 is exposed according to a circuit pattern. Coherent light (generally, laser light) L falls substantially vertically on a surface of the photoresist film 2 from above. A part of the light L is reflected on the surface of the photoresist film 2. Another part of the light L passes through the photoresist film 2, reaches the interface of the photoresist film 2 and the wafer 1 and is reflected thereon.
The light reflected on the surface of the photoresist film 2 is represented as light La. The light reflected on the interface of the photoresist film 2 and the substrate 1 is represented as light Lb. The light La and the light Lb are originally parts of the same light. Accordingly, the light La and the light Lb are able to interfere with each other and the interference wave changes dependent on the optical path difference .DELTA.l of the lights La and Lb. The optical path difference .DELTA.l changes when a developing solution is supplied to the photoresist film 2. This is because the thickness d of the photoresist film 2 decreases as the development by the developing solution proceeds.
When observed at a fixed point P on the optical paths of the reflected lights La and Lb, the intensity of the superposed light varies cyclically as a function of the film thickness. Consequently, the change of the thickness d of the photoresist film 2 can be detected by the variation of the intensity of the superposed light at the point P.
As the development proceeds, the thickness d of the photoresist film 2 decreases in pattern openings of the film 2 (where the photoresist is to be removed by the development). The intensity of the superposed light at the point P varies as the film thickness d decreases. When the photoresist film 2 is substantially removed to cause the openings to be penetrated, the intensity of the superposed light due to the decrease of the film thickness does not vary.
It is to be noted that even if the intensity of the superposed light due to the decrease of the film thickness ceases to vary, the intensity change of the superposed light at the point P continues, though slightly. This is because the measured value does not precisely represent the intensity of the superposed light only, due to various factors such as external disturbances received by the means for measuring the intensity of the superposed light. FIG. 2 shows such variations of the intensity of the superposed light at the point P.
Even after the decrease of the film thickness is ended, additional development is carried out for a prescribed amount of time in order to completely carry out the development. The period for the additional development needs to be accurately defined in order to carry out the development with good repeatability and with high efficiency. More specifically, if this period is too short, the development is insufficiently done. On the other hand, if this period is too long, excess development occurs and the efficiency of work is lowered.
Referring to FIG. 2, according to the conventional method, the point F where the variation of the light intensity becomes small (this point being hereinafter referred to as the variation decrease point) is used as a reference point for detecting an end point D of development. The point D where a preset time T.sub.0 of additional development has passed from the variation decrease point F is defined as the end point of development.
The method of detection of the variation decrease point F will be briefly described in the following. The intensity of the superposed light is measured every small unit of time. The difference between the presently measured and the previous values is obtained. Absolute values of the differences for 50 measurements for example are accumulated. The accumulated value is considered to represent the variation range of the intensity of the superposed light during the 50 measurements. The point where the result of the accumulation is smaller than a prescribed value is determined to be the variation decrease point F. Since this method is the same as a method of detection of a variation decrease point F used in an embodiment of the present invention, a detailed description thereof is set forth below.
The time t.sub.0 of additional development is determined for example in the following manner. A plurality of simulation wafers of the same specification as that of a wafer subjected to an actual development treatment (a wafer to be treated) are prepared. A photoresist film is formed on each simulation wafer under the same conditions as those for the wafer to be actually treated, and exposure according to a circuit pattern as well as other treatment is effected. The development treatment is applied to the respective simulation wafers for different periods of time with other conditions being the same.
Data on the intensity change of superposed light due to interference in each simulation wafer is obtained. Developed patterns of the respective simulation wafers are observed and compared with each other, and the simulation substrate having the best result of the development is selected. The end point of development of the selected simulation wafer is assumed to be D'. An end point F' of change of the superposed light is evaluated from the data on the intensity of the superposed light due to interference in the selected wafer. Based on the equation of F'+t.sub.0 '=D', the time t.sub.0 ' of additional development is calculated. The time t.sub.0 ' of additional development thus determined is also adopted in the actual development of a wafer to be treated.
However, the conventional method has the following problems. In determining the variation decrease point F, the degree of decrease in the variation of the intensity of the superposed light is empirically defined. Depending on the adapted value of the variation range for defining this point, the detection point of the variation decrease point F varies easily. In addition, the detected variation decrease point F is unstable due to errors in measurement. The variation decrease point F does not always accurately represent the point where the intensity of the superposed light ceases to vary due to the decrease of the film thickness.
Further, the superposed light behaves unstably when the photoresist is substantially removed in the pattern openings and the photoresist film 2 is penetrated. It is fundamentally difficult to correctly detect the end point of the development treatment based on decrease of the intensity variation of the superposed light. Consequently, according to conventional methods, the end point D of the development is often incorrectly detected. This results in deterioration of the repeatability of the development treatment and lowering of the efficiency of work.